The present invention relates generally to injection molding techniques and specifically to xe2x80x9csandwichxe2x80x9d molding in which a second material expands within a first material within a mold cavity.
In conventional injection molding, a thermoplastic material is injected from an injection nozzle into a mold cavity formed by two or more mold portions. The thermoplastic material passes from the injection nozzle along a runner passage in the mold, through a gate (normally a constricted portion of the runner passage), and into the mold cavity.
When the thermoplastic material has cooled enough to solidify, the mold portions are separated and the molded part and a xe2x80x9crunnerxe2x80x9d formed in the runner passage are ejected. The ejection process may break the runner from the molded part or this may be done in a separate xe2x80x9cdegatingxe2x80x9d step.
In sandwich injection molding, two materials are injected one after another into the mold cavity. By proper control of the injection process, the second material expands inside the first material so that the first material envelops the second material in a xe2x80x9csandwichxe2x80x9d. Sandwich molding is useful when different properties are required in the materials forming the center and outer surface of the molded part. For example, a recycled plastic material may be used in the center of the part while the outer surface may use a material having improved color, surface finish, and opacity.
In the molds for sandwich molding, the runner passage is normally heated to keep the first thermoplastic material from hardening and blocking the second thermoplastic material. The heating may be accomplished by use of a heated manifold joining streams from two injection nozzles to a common injection point on the mold or through a xe2x80x9chot runner systemxe2x80x9d being heated passageways built into the mold. The manifold or hot runner system may include valves in the runner passage to prevent backflow of the material into the branch not being used for injection of material. The heated runner or manifold prevents material from solidifying on the valves rendering them inoperable.
Using a hot runner system or heated manifolds significantly increases the cost of manufacturing a mold and may be impractical for low cost or short run sandwich molding projects.
The present inventors have discovered that proper design of the runner passage can allow sandwich molding in which material is injected alternately in different branches of an unheated runner passage. The check valves that might be expected to clog if unheated are eliminated by holding the injection nozzles on each branch of the runner passage thereby creating sufficient pressure to prevent back flow to that runner branch. The result is a low cost mold that expands the availability of sandwich molding techniques.
More specifically, the present invention provides a method of sandwich molding comprising the steps of joining mold portions at a part line to define the mold cavity and a branched runner passage, the latter providing an unheated path connecting the mold cavity through a gate portion to a first and second branch exiting from a first and second opening in the mold. A first and second injection nozzle are abutted against the first and second openings, respectively, and a first thermoplastic material from the first injection nozzle is injected through a portion of the runner passage and gate to the mold cavity. Subsequently, a second thermoplastic material from the second injection nozzle is injected through a second branch of the runner passage and gate to the mold cavity to expand within the first thermoplastic material in the mold cavity. The first and second thermoplastic materials are allowed to solidify in the mold cavity and runner passage and the mold portions are opened and the molded part is separated from a runner formed in the runner passage.
It is thus a principal object of the invention to provide a simple molding technique for sandwich molding that does not require heated runners or the like.
The runner passage may extend along the part line or may have one branch extending along the part line and a second branch of the runner extending perpendicularly to the part line.
Thus, it is another object of the invention to provide simple variations for producing a branch runner passage consistent with extracting the solidified runner after the molds are separated.
The first injection nozzle may be maintained in position against the mold to block backflow of the second thermoplastic material from the first opening during the injection of the second thermoplastic material. Likewise, the second injection nozzle may be maintained and positioned against the mold to block backflow from the first thermoplastic material from the second opening when the first thermoplastic material is injected.
Thus, it is another object of the invention to eliminate the need for valves such as would necessitate a heated runner system.
The foregoing and other objects and advantages of the invention will appear from the following description. In this description, reference is made to the accompanying drawings, which form a part hereof, and in which there is shown by way of illustration, a preferred embodiment of the invention. Such embodiment and its particular objects and advantages do not define the scope of the invention, however, and reference must be made therefore to the claims for interpreting the scope of the invention.